Propeller manufacture



p i12s, 1936. F. w. CALDWELL 2 039,211

PROPELLER MANUFACTURE Filed June 23, 1933 INVENTOR Frank W [Ia/17m? mflaf u.

ATTORNEY Patented Apr. 28, 1936 UNITED STATES PROPELLEB MANUFACTUREFrank W. Caldwell, West Hartford, Conn., assignor, by mesne assignments,to United Aircraft Corporation, a corporation of Delaware ApplicationJune 23,

17 Claims.

This invention relates to improvements in propeller manufacture.

An object of this invention is to produce a propeller in which thecenter line of the blade end passes through the center of gravity of theblade.

Another object consists in provision of a means and a process forreducing the labor required, and otherwise facilitating the productionof propeller blades which shall be in dynamic and static balance invarious pitch settings.

Further and other objects and advantages will be apparent from thespecification and claims and from the accompanying drawing whichillustrates what is now considered the preferred embodiment.

Fig. 1 is an elevation, showing the means and method of locating a linethrough the center of gravity of a propeller blade.

Figs. 2 and 3 show means for suspending a blade of another type.

Fig. 4 is a perspective view of balancing and checking means.

This invention is particularly applicable to propellers of thedetachable blade type, such as those shown in Patents Nos. 1,608,755;1,769,775 and 1,836,700. In this type of propeller, the blades aredetachably secured in a hub and are adapted to be adjusted in the hub togive various a0 pitch settings. The portion of the blade which issection and is called the blade-end and in turning the blade in the hubto secure a pitch adjustment, the entire blade is turned about thelongitudinal center line of the blade-end.

By shaping the blade-end so that its longitudinal center line passesthrough the center of gravity of the blade, the dynamic forces of theblade will always pass through the same point of the blade hub so thatpitch adjustment will not vary the dynamic balance of the blade.

Furthermore, a set of blades in which the longitudinal center line ofthe blade-ends passes through the center of gravity of the respectiveblades, when secured and statically balanced in a properly machined hub,will be found to be in dynamic balance and will remain in dynamicbalance for various pitch settings. This is necessarily so because ifthe propeller is statically balanced, the dynamic forces are necessarilyequal, and if the center of gravity of each blade lies on thelongitudinal center line of the blade end, the equal dynamic forces allact in a single plane normal to the axis of the shaft in which the hubis mounted and there are, theresecured in the hub is of circulartransverse cross- 1933, Serial No. 677,201

fore, no unbalanced forces to give dynamic balance. 3

In the drawing, a pair of brackets I0 and I 2 are secured in verticallyspaced relation upon any suitable means such as a wall or post It. 6 Thebracket Ill is provided with an aperture it which is covered by a platei8, which plate is held in adjusted position by means of screws 20. Theplate i8 is provided with enlarged holes 22 to permit adjustment.

The bracket I2 is provided with a bushing 24 in which a prick punch 26is slidably retained.

One end of a wire 28 passes through the center of an externally threadedplug 30, and is secured thereto as by brazing and an externally threadedl5 plug 32 is similarly attached to the other end of the wire. Theseplugs are adapted to be threaded into plate l8 and propeller blade 34respectively so that the propeller blade may be suspended by the wire 28from the plate I3.

In case the propeller blade end is hollow, as shown in Figs. 2 and 3, acircular plate 36, having screws 38 extending radially therefrom, isadapted to be secured in the opening in the bladeend by backing thescrews 38 out until they 25 firmly engage the blade-end. The plate 36has a threaded hole in the center thereof which is adapted to receivethe plug 32 and is also countersunk to receive a lathe center.

In order to locate the center of gravity of a 30 propeller blade uponthe center line of the blade end, a propeller blade is shaped tosubstantially the contour desired in the finished article with the bladeend, however, left slightly oversize. The propeller blade isthensupportedin a lathe 35 or other suitable machine, and in the type ofpropeller shown in Fig. 1, a hole is threaded and tapped at the centerof the blade-end. This may be conveniently done byclamping a block tothe blade-tip to receive the head center of the 40 lathe and supportingthe blade-end in a steady rest.

In the type of propeller shown in Figs. 2 and 3, the blade maybesupported between centers in the lathe, the hole in the center ofplate 36 serv- 45 ing to support the propeller on one of the lathecenters. The plate 36 is then adjusted by means of the screws 38 untilthe outside of the bladeend runs true.

Plug 32 is then screwed into the accurately centered tapped hole in theblade-end, and plug 30 is screwed into the tapped hole in plate l3. Thistapped hole in plate l3 has previously been accurately centered directlyabove the center of the prick punch 36, by suspending a plumb bob 55from the center of the hole in plate I8 and ad- Justing the plate untilthe plumb hob and the prick punch register.

The'propeller is allowed to hang freely from the wire 28 which passesthrough the center of the blade-end. The center of gravity of the blade.indicated by the letters C. G. on the drawing, will be directly underthe point of suspension and as the prick punch 26 is located directlyunder the point of suspension, the prick punch point, the center ofgravity of the blade and the center of the blade-end, from which theblade is suspended, will all fall in the same straight line. The prickpunch may now be hit with a hammer to locate the point on the blade tipwhere this imaginary line emerges.

If this point should be outside of the blade tip, a suitable block maybe fastened to the propeller tip to receive the prick punch mark. If itbecomes necessary to add this block, care should be taken to make surethat the propeller blade is in the same lateral position at the time themark is made, that it was, when it was hanging free without the additionof the block. This may be accomplished either by clamping the blade inits freely suspended position and then adding the block, and prickpunching it or adjusting weights on the block so that the propellerassumes the same position after the addition ,of the block that it hadbefore the addition of the block.

After the center has been located and marked at the propeller blade tip,the blade is hung between centers in a lathe or other suitable machineon the line connecting the center of the blade-end and the center markedat the blade tip and passing through the center of gravity. Theblade-end may now be machined around this line to fit its supporting hubwith the assurance that the center of gravity will lie on thelongitudinal center line of the blade-end. As this is substantially thelast operation to be performed upon the blade as far as any operationsthat would affect the center of gravity are concerned, as the blade haspreviously been brought to the correct size and shape, the center ofgravity of the finished blade will still be on the longitudinal centerline of the blade-end of the finished blade. Any finishing operations,such as bufling and pollishing that may be later performed, will haveeither no or only a negligible eifect upon the location of the center ofgravity.

The above described process is of particular advantage in producing apropeller such as that described and claimed in my Patent No. 1,769,767,July 1, 1930. After a blade of the desired contour has been produced sothat the centers of gravity of all the sections lie on a straight lineas called for by that patent, the use of the above described processwill enable the machining of the blade end with the assurance that thecenter line of the blade and will pass through the center of gravity ofthe blade. For all practical purposes, in a properly designed blade,this center line will then pass through the center of gravity of all theblade sections.

The position of the center of gravity in the finished blade may bechecked in the following manner; the blade 34 is mounted in one socketof a holder which is preferably a two way hub 40 and an accurately sizedcylinder 42 is mounted in the other socket The hub with the bladestherein is then mounted on a shaft 44 which in turn is placed onbalancing knife edges 46. The blade 34 is so mounted that it may beturned in the hub socket and about its longitudinal axis through 360.The cylinder is chosen of such a size that its statical moment about thehub support is the same as the statical moment of the blade. The centerof gravity of the cylinder will fall on the center line bf the cylinderand hence serves as an accurate and simple means of checking theposition of the center of gravity of the blade. The cylinder 42 iscommonly referred to as the master cylinder, or if an accurately sizedand finished blade is used instead of the cylinder for determining thebalance of the newly finished blades, this accurately sized and finishedblade is known as the master blade and indeed both the master cylinderand the master blade are often referred to simply as master.

The blade is balanced in various positions including horizontal andvertical positions. In the vertical position the blade is turned bysteps or intervals through 360 in the hub socket. It is, of course,theoretically possible to determine the balance of the blade by movingit by steps through 180 providing the master is perfect. Movementthrough 360 will also, however, indicate any inaccuracy of the master orthe hub. If the center of gravity of the blade is on the center line ofthe blade end, the blade will be in balance with the cylinder in all thevarious positions of the blade in the hub socket.

If, however, the center of gravity is of! of this center line and theblade is turned in the socket to a position such that a plane containingthis center and the center line of the blade end is at an angle to theplane containing the center line of the blade end and cylinder and thecenter line of the shaft 44, a condition of unbalance is created. Anyvariation of the center of gravity from the center line of the blade endcan thus be readily checked and corrected. It is to be understood thatthe invention is not limited to the specific embodiment or steps hereinillustrated and described, but may be used in other ways, and the stepsperformed in a different sequence without departure from its spirit asdefined by the following claims.

What I claim is:

1. In the manufacture of a propeller blade the steps of shaping theblade to substantially the shape and size desired in the finished blade.locating a point at substantially the center of the blade-end, freelysuspending the blade from that point so that the center of gravity ofthe blade will be directly under the point of suspension, marking apoint at the blade tip of the suspended blade substantially on thecontinuation of the straight line passing through the point ofsuspension and the center of gravity of the blade, supporting the bladeupon the line between the said points at the blade-end and at the bladetip, and machining the blade-end while so supported so that thelongitudinal axis of the blade-end will'coincide with said line.

2. The method of forming a propeller blade which includes shaping theblade to substantially the shape and size desired in the finished blade,marking a point at the blade tip substantially on the continuation ofthe line connecting a point in the center of the blade-end and thecenter of gravity of the blade, supporting the blade at the center ofthe blade-end and at the point marked at the blade tip, and forming thebladeend so that its longitudinal axis coincides with said line.

3. In producing a propeller blade the steps of shaping the blade portionto substantially finished size and shape,supporting the blade on a linepass-

